Could be one of two faults:
1) Knackered switch or poor wiring at the switch connector (not very likely, but still possible)
2) Fan resistor pack, usually located near the fan mechanism itself, which in turn is usually located behind the glove box. (First place I'd look if I were you)
A replacement fan resistor pack can be bought from any Renault service center worth their salt, if you want to be 110% sure you've got the real one.
If you want to get a replacement at the scrappy or on eBay instead, remove the one you've got,
so you have something to compare with when looking at the replacements the scrappy can offer or photos on eBay.